1. Field of the Invention
The present invention relates to fuel-barrier polyamide resins and multilayer shaped articles, and more particularly, relates to polyamide resins that are minimized in the fuel permeability, excellent in heat resistance and moldability and suitably used as a material for fuel containers, and further relates to multilayer molded articles using the polyamide resins.
2. Description of the Prior Art
Polyamide resins have been extensively used in applications such as not only injection-molding materials for automobiles and electric or electronic parts, but also packaging materials for foodstuffs, beverages, drugs and electronic parts, because of their excellent mechanical properties. Among the polyamides, polyamides produced by the polycondensation of xylylenediamine and aliphatic dicarboxylic acid (hereinafter occasionally referred to as “MX nylon”), especially polyamides produced from m-xylylenediamine and adipic acid (hereinafter occasionally referred to as “polyamide MXD6”), exhibit a low permeability to gaseous substances such as oxygen and carbon dioxide gas and, therefore, have now been used as gas-barrier materials for shaped articles such as films and bottles.
In recent years, from the viewpoints of reducing the weight, dispensing with rust-proofing treatment, making design liberty greater, reducing the number of steps and making production process fully automated, resin-made fuel containers produced by blow-molding, etc. have been noticed as fuel storage containers, and increasingly used as a substitute for metal-made fuel containers.
Polyethylene (high-density polyethylene) conventionally used for the fuel containers is excellent in mechanical strength and moldability, and inexpensive, but fails to meet recent regulations on fuel permeation because of its poor barrier property to fuels.
To solve the above problems, there have been proposed a method of subjecting an inside surface of the containers to fluorine treatment, and also proposed multilayer containers having an intermediate barrier layer made of a fuel-barrier polyamides or ethylene-vinyl alcohol copolymer between polyethylene layers (for example, Japanese Patent Application Laid-Open Nos. 3-32815, 5-345349, 6-340033, 9-29904 and 2001-97053). Among these proposals, the fluorination treatment has problems such as less safety due to the use of harmful gases and difficulty in recovery after the treatment, and therefore has been rarely employed at present. The proposed multilayer containers are somewhat successful in reducing the fuel permeability because of the barrier layer, but fail to show a fully satisfactory fuel-barrier property. If the thickness of the barrier layer is increased, there tend to occur problems such as deteriorated shock absorption upon collision as well as increase in weight and costs. Therefore, such multilayer containers are difficult to fully meet the regulations that will be made stricter. Also, the use of ethanol as fuel has been recently studied and put into practice, because the addition of ethanol to gasoline allows to reduce the use of fossil fuel as well as reduce the emmision of carbon dioxide gas. However, nylon 6, ethylene-vinyl alcohol copolymers, etc. are poor in barrier property to alcohols. Therefore, it has been strongly demanded to provide materials having a higher barrier property to fuels and alcohols.
The fuel containers are generally produced by a direct blowing method. Since the known barrier materials are poor in heat resistance, the gelation is likely to occur, thereby limiting the production conditions. In addition, there are various problems in the productivity and the effective use of materials because of the need of recycling the flash, formed during the molding operation and the need of changing into a resin with more higher thermal stability when the machine is started and stopped. In particular, in the direct blow-molding method, the use of resins having a low melt viscosity leads to occurrence of draw-down, which results in problems such as the production of molded articles having an excessively small wall thickness and uneven wall thickness. Further, the condition of an excessively high molding temperature tends to lower the viscosity of molten polyolefins for the outer layer of the fuel containers, thereby promoting occurrence of draw-down.